The demand for smaller electronic components with higher performance and reliability is continuing to grow as equipment becomes smaller, thinner, and lighter, and electrical equipment circuits become more densely packed and digitized. There is, as a result, an accelerating demand for capacitors with the characteristics of smaller size, larger capacitance, and lower impedance at high frequencies.
Capacitors with low impedance at high frequency include ceramic capacitors, which employ ceramics as their dielectrics, and film capacitors, which employ organic polymer film as their dielectrics. However, to achieve large capacitance, these types of capacitors require a larger size accompanied by a proportional increase in cost.
On the other hand, electrolytic capacitors, which employ aluminum or tantalum oxide film as their dielectrics, can achieve large capacitance with small size; however, their impedance and dielectric characteristics in the high frequency band degrade comparing with those of the aforementioned ceramic capacitors and film capacitors. Therefore, to improve their high frequency characteristics, aluminum solid electrolytic capacitors have been developed and made available in the field of aluminum electrolytic capacitors. The aluminum solid electrolytic capacitor employs as electrolyte solid materials which have higher conductivity than conventional materials, such as tetra-cyano quino-di-methane (TCNQ) complex and polypyrrole. Also in the field of tantalum electrolytic capacitors, polypyrrole, which has higher conductivity than manganese dioxide, is used for the cathode, and is now available on the market.
The above electrolytic capacitors have significantly improved high-frequency characteristics because the equivalent serial resistance (ESR) of capacitors is lowered by adopting highly conductive compounds as the cathode matrials.
Also in the field of film capacitors, attempts have been made in recent years to make small the capacitor and increase its capacitance without losing the good dielectric characteristics of organic high polymer films. (Refer to Japanese Laid-open Patent Nos. S63-29919 and H3-203211.)
For example, the Japanese Laid-open Patent No. H4-87312 discloses a method for manufacturing a new type small film capacitor with large capacitance. Instead of forming an aluminum oxide film on a surface of etched aluminum foil having a large surface area, the new type capacitor employs a dielectric layer of polyimide film made by electrodeposition of polyamic acid solution. And over the polyimide film conductive high polymers are formed.
Furthermore, the Japanese Laid-open Patent No. H9-115767 discloses a method for manufacturing another new type of small film capacitor with a large capacitance. Instead of forming an aluminum oxide film on a surface of etched aluminum foil, this new type capacitor employs an aqueous solution containing polyacrylic acid derived resin (acrylic resin), which is amenable to industrial processes, to form a dielectric layer of thin polyacrylic acid derived resin film by means of electrodeposition, after which conductive high polymers are formed on the surface of the dielectric layer.
These inventions improve capacitance per unit volume while maintaining the high insulation characteristics, non-polar characteristics, and low dielectric loss factor (tan .delta.) of polyimide or polyacrylic acid derived resin used as dielectrics, the advantageous properties characteristic of organic high polymer films.
However, th ere are two major concerns with the above conventional technologies. The first concern which the present invention aims to solve is described below.
In the Japanese Laid-open Patent No. H4-87312, an organic solvent such as dimethyl formamide (DMF) or methanol is used as the solvent for the polyamic acid salt solution. For safety reasons, application of voltage to electrodeposition solutions containing large amounts of organic solvents is not recommended. In addition, it is necessary to take into account the toxicity to the human body of these organic solvents. Moreover, since the solution costs more than water, it is not suitable for industrial mass production. Furthermore, the polyamic acid film formed by electrodeposition requires to be heat treated at above 200.degree. C. to imidize the film.
Polyamic acid, when exposed to a high-humidity atmosphere, easily decomposes and shows low stability in storage. Accordingly, the molecular weight may deviate even when polyamic acid film is electrodeposited and then imidized by heat treatment. The durability of the film is also poor. Since the film quality is not ideal, capacitors using polyimide film created by heat treating polyamic acid electrodeposited film show deviations in dielectric characteristics. In addition, the capacitance of polyimide film drops considerably over time if voltage is continuously applied at temperatures between 80 and 200.degree. C.
The invention disclosed in Japanese Laid-open Patent No. H9-115767 employs aqueous polyacrylic acid solution as the electrodeposition solution, which is better suitable to mass production. However, the heat resistance of the polyacrylic acid electrodeposition film thus formed is not as high as that of the polyimide film. In addition the thickness of the film tends to become thicker than that of the polyimide film, which results in smaller initial capacitance, even if the dielectric is formed by applying the same electrodeposition voltage.
The present invention solves the above problems with conventional techniques and aims to provide a small capacitor with high productivity, non-polar characteristics, and large capacitance, and its manufacturing method by adopting a new polyimide as a dielectric material.
Next, the second concern which the present invention aims to solve is described.
In response to the demand for smaller electronic equipment and to the demand for high frequency switching power supply, attempts are being made to increase the capacitance of aluminum solid electrolytic capacitors using TCNQ complex or polypyrrole for the cathode by rolling or laminating etched aluminum foil.
If etched aluminum foil is rolled, mechanical stress is applied to the bent portions, which may damage the oxide film and degrade its electrical characteristics.
Even if the foil is laminated, the oxide film of the dielectrics is thin and prone to mechanical stress, risking damage to the oxide film during the lamination process, causing defects. Accordingly, it is considered difficult to laminate elements by applying pressure. As the number of laminated layers increases, the defect rate due to larger leak currents tends to increase in proportion. Therefore, it may be difficult to increase the rated voltage to achieve a large capacitance by increasing the number of laminated layers.
Other causes of the above problem include poor oxide film recovery capability of solid electrolyte such as polypyrrole, compared to liquid electrolyte, and difficulties in eliminating defects on the oxide film during anodization.
Accordingly, it may be difficult to increase capacitance by simultaneously increasing the number of laminated layers and the rated voltage for solid electrolytic capacitors that employ an oxide film dielectrics.
In addition, although the frequency characteristics of large capacitance solid electrolytic capacitors, including tantalum solid electrolytic capacitors, as mentioned above have been significantly improved by lowering their ESR, they still do not match the desirable characteristics of film capacitors. Furthermore, solid electrolytic capacitors may present problems if used in AC circuits and circuits to which reverse voltage is applied, because the oxide films of dielectrics possess polarity.
The present invention solves the above conventional problems, and provides a new type small laminated capacitor with good frequency characteristics equivalent to those of film capacitors, small leak current, non-polarity, and large capacitance. Its manufacturing method is also provided.